Advancement in technology of semiconductor devices provisioned to operate the semiconductor devices at higher switching frequencies. Sizes of energy storage elements associated with the semiconductor devices may decrease linearly with increase of switching frequency. Therefore, high density printed circuit demands high switching frequency and faster semiconductor devices. The increase in switching frequency together with increased current slew rates and voltage slew rates i.e.,
      di    dt    ⁢          ⁢  and  ⁢          ⁢      dv    dt  respectively, have damaging effects on Electro Magnetic Compatibility (EMC) in the semiconductor devices. These damaging effects have led to deployment of spectrum of conducted and radiated frequencies at higher frequencies in the semiconductor devices. Electro Magnetic Interference (EMI) filter is generally employed to decrease the damaging effects on the EMC. Also, the filter typically has degraded attenuation due to stray elements at high frequencies, and is less effective in filtering out high frequency Electro Magnetic interference (EMI) noises which are generated from the semiconductor devices. Also, such filters use large space on Printed Circuit Board (PCB) that makes it bulky.
FIG. 1 illustrates a conventional PCB 100 with two differential lines. The conventional PCB 100 illustrated is a single layered PCB comprising two differential lines namely first differential line 103 and second differential line 104. Each of the two differential lines are placed on different planes, namely first plane 101 and second plane 102 of the PCB 100. The first plane 101 and the second plane 102 are conductive surfaces of the PCB 100. The first differential line 103 is placed on the first plane 101 and the second differential line 104 is placed on the second plane 102. The first differential line 103 carries forward current and the second differential line 104 carries backward current. Further, the forward current and the backward current comprise differential mode current and common mode current. The differential mode current and the common mode current, as the forward current, flow in forward direction in the first differential line 103. Whereas, the differential mode current as the backward current flows in backward direction and the common mode current as the backward current flows in forward direction in the second differential line 104. Thereby, the differential mode current in the differential lines 103 and 104 flow in opposite directions with respect to each other and the common mode currents in the differential lines 103 and 104 flow in same directions with respect to each other. Due to the common mode current, there arises EMI in the PCB and affects the EMC.
Conventional systems in the art disclose one or more techniques for reducing the EMI and increase EMC in the PCBs. One of the techniques includes one or more methods for reducing common mode current by which the EMI is reduced in the PCB. Thereby, the EMC of the PCB is enhanced.
One of the methods for reducing the common mode current at the PCB level includes creating hollow geometry on ground plane which is placed adjacent to a signal layer of the PCB. The hollow geometry may be a spiral structure 200 as shown in FIG. 2. The hollow geometry 200, on the ground plane acts like a common-mode choke in the PCB. As described in FIG. 1, the signal layer comprises two differential lines which are placed in a way to create a common mode filter at the PCB. However, due to a dielectric layer placed between the signal layer and the ground layer, stray capacitance is formed and thereby the said method may not be efficient as the efficiency depends upon thickness of dielectric layer.
Another method discloses to reduce common mode current at the PCB level by creating spiral geometry 200 on phase line signal layer and neutral line signal layer of the PCB. In one embodiment, the spiral geometry may be the spiral structure 200 as illustrated in FIG. 2. The said layers are separated using dielectric material which acts as a capacitor filter. Thereby the common mode current is reduced in this method. However, there is a need of ground terminal for reducing common mode current. Further, the hollow space formed due to the spiral structure 200 has dielectric media, which may create inter-winding capacitance and may create several smaller size loops within the spiral structure 200. Also, absence of high permeability in the PCB reduces magnetic flux linkage from phase to neutral which is generated by high frequency common mode current.
In the said methods for reducing the common mode current, inserted loss of the differential mode current due to additional careless spiral structure is higher. The said inserted loss is undesired. Further, to form multiple resonance frequencies into single PCB, there is a need for more number of hollow geometries such as the spiral structure 200. Also, the said method requires copper made ground plane layer which considerably increases cost and size of the PCB. Further, space used for creating a common-mode filter at the PCB level in the said methods is larger.